Our earlier findings on the alterations of tRNA populations of hepatomas and fetal liver have been confirmed with the more sensitive system of reverse phase chromatography. We are extending these studies to other aminoacyl tRNAs of fetal liver and we are also examining the nature and function of the multiple species of aminoacyl tRNAs for histidine, asparagine, and tyrosine. We are continuing our studies on the biological function of polyamines. Polyamines do not appear to stimulate directly the synthesis of nucleic acids as previously proposed. The results obtained suggest an indirect or secondary role for spermidine in which the polyamines stimulate stable RNA synthesis probably by relieving RNA product inhibition of RNA synthesis. By studying the growth of RNA bacteriophage f2 in a conditional putrescine auxtroph of E. coli, we have obtained direct evidence for a physiological role for these organic cations in normal protein synthesis.